Among all described serotonin (5-HT) receptors in mammals, the type three (5-HT3) is the only ion ligand gated channel receptor for serotonin. By using double in situ hybridization histochemistry, we found high levels of co-expression of the functional 5-HT3A subunit of the 5-HT3 receptor and the central CB1 cannabinoid receptor in neurons of the rat telencephalon. Double-labeled 5-HT3A/CB1 neurons were found in anterior olfactory nucleus, superficial and deep layers of cortex, hippocampal formation (hippocampus, dentate gyrus, subiculum and entorhinal cortex) and basolateral amygdala. Analysis of the proportion of 5-HT3A neurons coexpressing CB1 receptors demonstrated that, depending of the forebrain region, 50-80% of all neurons expressing the 5-HT3A subunit also express CB1 transcripts. However, an even higher proportion of CB1-labeled neurons contains 5-HT3A mRNA. About 75-90% of all CB1 expressing neurons co-express the 5-HT3A subunit in cortical areas, and the majority of CB1 positive neurons have 5-HT3A mRNA in the basolateral amygdala. By using a combination of double in situ hybridization and immunohistochemistry, we further confirmed that 5-HT3A/CB1 expressing neurons contain the inhibitory neurotransmitter g-amino butyric acid (GABA). These results suggest that in distinct regions of the telencephalon, GABA neurons that react to cannabinoids may also be responsive to serotonin through 5-HT3 receptors. Cellular coexistence of 5-HT3 and CB1 receptors in interneurons of cortex, hippocampal formation and amygdala suggest possible interactions between the cannabinoid and serotonergic systems at the level of GABA neurotransmission in brain areas involved in cognition, memory and emotion.